1. Field of the Invention
The present invention relates to an actuator having a flat spring structure including a vibratingly moving plate, which is manufactured by micromachining using semiconductor-manufacturing technology.
2. Description of the Related Art
Since the above actuator can be manufactured very small, it is often used for manufacturing micro-sized optical scanners. The optical scanner includes a vibratingly moving plate, and the moving plate has a mirror for reflecting a light beam. As the moving plate moves, the optical scanner scans the light beam reflected by the mirror.
A typical actuator for use in an optical scanner that performs two-dimensional scanning has a so-called gimbal-type flat spring structure, which comprises a moving plate supported vibratingly about two axes. In other words, the flat spring structure includes a moving plate, a moving inner frame surrounding the moving plate, a pair of first torsion bars coupling the moving plate and moving inner frame so as to allow the moving plate to be vibrated relative to the moving inner frame, a fixed outer frame surrounding the moving inner frame, and a pair of second torsion bars coupling the moving inner frame and fixed outer frame so as to allow the moving inner frame to be vibrated relative to the fixed outer frame.
In the foregoing actuator, the moving plate is usually vibrated by electromagnetic driving or electrostatic driving. In an electromagnetic driving type actuator, a moving plate and a moving inner frame include an electromagnetic coil. A permanent magnet applies a magnetic field to the electromagnetic coil. In an electrostatic driving type actuator, a moving plate and a moving inner frame include a pair of driving electrodes, respectively, and a fixed electrode is provided opposite to the driving electrodes.
A new technique, in which a laser projector uses the above optical scanner, is known. Basically the laser projector includes a light source for emitting a light beam, and an optical scanner for scanning the light beam two-dimensionally.
The laser projector requires an about-three-digit difference between the horizontal scanning frequency and vertical scanning frequency. In the flat spring structure of the two-dimensional driving actuator used in the optical scanner, therefore, the first and second torsion bars need to have greatly different resonance frequencies.
In a prior art flat spring structure manufactured by micromachining, first and second torsion bars are usually made of the same material. The resonance frequencies of the first and second torsion bars are inevitably similar to each other. Thus, a two-dimensional driving actuator using the prior art flat spring structure cannot meet laser projector design requirements.
The optical scanner preferably has a function of detecting a two-dimensional inclination of the moving plate or a deflection angle. In the electrostatic driving type optical scanner, a two-dimensional inclination of the moving plate is obtained by measuring the capacitance between the fixed electrode and the four driving electrodes.
In a prior art electrostatic driving type actuator, generally, driving electrodes are provided on the moving plate and the moving inner frame, and a member that supports the flat spring structure is formed of a silicon substrate, which serves as an electrode. The driving electrodes are electrically connected to an external circuit through wires formed on the silicon substrate with a thin insulation film interposed therebetween. Due to this arrangement, parasitic capacitance is caused between the wire and the silicon substrate, which decreases the measurement precision of capacitance or detection precision of inclination of the moving plate.